Comparison of disseminated and nondisseminated strains of Borrelia burgdorferi sensu stricto in mice naturally infected by tick bite

Abstract

Clinical isolates of Borrelia burgdorferi sensu stricto have been categorized into disseminated and nondisseminated groups based on distinct ribosomal spacer restriction fragment length polymorphism genotypes (RSTs). In order to determine whether transmission by tick bite would alter the dissemination dynamics and disease produced by distinct genotypes, disseminated isolates (RST1), nondisseminated isolates (RST3), and a standard laboratory strain (B-31) were established in a murine cycle utilizing infections transmitted by ticks. B-31 spirochetes circulated in the blood of inbred C3H/HeJ mice longer than in the blood of outbred mice. The majority of C3H mice exposed to RST1-infected ticks contained cultivable spirochetes in their blood for up to 17 days; in contrast, mice exposed to RST3 isolates demonstrated a precipitous decline in infection after day 7 postexposure. A quantitative PCR (q-PCR) assay demonstrated that the densities of spirochetes in blood were similar for the RST1 and RST3 isolates, except during the 2nd week postexposure, when the RST1 isolates displayed a markedly higher density in blood. Spirochete load in the heart and bladder of infected mice was measured by q-PCR at 8 weeks postexposure; the numbers of spirochetes in these tissues were similar for mice infected with either disseminated or nondisseminated strains. Similarly, histopathology samples of heart, bladder, and joint tissue obtained at 8 weeks postexposure did not reveal greater pathology in mice infected with the disseminated isolates. We conclude that although the spirochetemia induced by tick-transmitted disseminated isolates was more intense and of longer duration than that induced by nondisseminated isolates, the resultant pathologies produced by these strains were ultimately similar.

FIG. 1.

Percentages of infected C3H/HeJ mice (number of culture-positive/number of mice examined) versus outbred mice over a 1-month period. The time points shown represent the days after the last tick fed to repletion. Mice were naturally infected by B-31-infected nymphal tick bite, and ∼200 μl of whole blood was cultured in BSK-H medium to detect infection with viable spirochetes. Significant differences (P ≤ 0.05) in infection rates among C3H and outbred mice occurred at days 0, 14, and 17 (*).

FIG. 2.

Culture of RST1 strain 142 and 154 and RST3 strain 151 and 173 spirochetes in BSK-H medium from whole blood of 12 C3H/HeJ mice naturally infected by tick bite. The percentage of culture-positive mice (number of culture-positive/number of mice examined) for nine time points over a 1-month period for each of four strains is represented. The time points shown represent the days the last tick fed to repletion. Significant differences (P ≤ 0.05) between the combined RST1 versus RST3 proportion that was culture positive occurred on days 10, 14, and 17 (*).

FIG. 3.

Number of spirochetes in whole blood (0.1 ml) through 1 month as determined by TaqMan for RST1 strain 142 and RST3 strain 151. Each time point for each strain represents data for two mice.